The writers describe a rare case of rupture with disinsertion of the proximal end of the adductor longus tendon. The morphological and histological observations made on a large fragment removed from the terminal portion of the tendon showed a marked increase in the thickness of the fibro-cartilage at the level of the insertion. On the basis of these observations, the hypothesis is put forward that one of the possible pathogenic mechanisms of tendon rupture-disinsertion is an increase in the thickness of the fibro-cartilage of the tendon at its insertion into the bone, with consequent diminution of mechanical efficiency.